EHS36 with Plantronics Savi720 on T38G

[stscc]

We just purchased 4 T38G IP-phones with EHS36 adaptors to connect Plantronics Savi 720 headsets. We can't get the headsets to work properly. Depending on the setting of the switch on the base of headset incoming callers hear me but I don't hear them or the other way round. If I put the switch in position "C", everything works, I can pick up incoming calls from the headset, the icon on the phone indicates a connected headset, but the microphone of the headset is dead.
Plantronics support stated position "A" would be correct, but I doubt that, since in this position the headsets don't work at all. They didn't want to make any comments on the EHS36 since it is a third-party product.
Any ideas anyone...?
Can I perform a firmware upgrade on the EHS36 using the T38G phones? I only found firmware files for use with the T26/T28 phones. How can I determine the firmware version of the EHS36?